Research Cruise Pinpoints Phytoplankton’s Critical Role In Marine Phosphorus Cycle

Phytoplankton, or sea sawdust, play an an important role in the marine phosphorus cycle. (Credit: NOAA MESA Project)

Phosphorus is a nutrient as vital to life as it is ubiquitous, yet scientists know relatively little about how it cycles through the environment or changes its oxidation state.

Researchers from the Woods Hole Oceanographic Institution and Columbia University studied phytoplankton to discern how two distinct phosphorus molecules relate to one another in the ocean, revealing a more complex marine phosphorus cycle than previously known.

The study also revealed that cyanobacteria and other classes of plankton play an important role in phosphorus cycling. A paper detailing the study’s findings is published online in Science.

“We knew there were two types of phosphorous molecules in the ocean, but we had no idea how they were connected,” said Benjamin Van Mooy, the paper’s lead author and associate scientist at WHOI.

Ocean phosphorus most often manifests in one of two oxidation states, determined by the gain or loss of electrons.

The study focused primarily on the decomposition rate of phosphate in its +5 oxidation state to the nutrient in its +3 state. But the decision to target that measurement came late in the process, according to Van Mooy.

“We set off to measure a bunch of things, and this measurement that was the topic of the science paper was kind of a last minute thing; it just occurred to me like a lightning bolt,” Van Mooy said, noting that, when it comes to research cruises, a decision made a month before departure counts as last-minute.

Sailing from Bermuda to Barbados, the research cruise took on a cross-discipline ebb and flow, Van Mooy said, starting each day with biology and ending it with analytical chemistry.

The researchers sampled phytoplankton every morning at sunrise for several weeks. They used nets to strain the so-called “sea sawdust” out of the water, where the organisms form blooms large enough to see from space.

During the day, the researchers ran experiments aboard the vessel, adding radioactive phosphate to samples to isolate certain molecules. They found colonies of nitrogen-fixing phytoplankton significantly contribute to phosphorus’ chemical reduction from the +5 state to its +3 state.

Despite the scenic locale, the research cruise favored work over play, and exhibited the same challenges as any close-quarters job.

“Working at sea is amazingly challenging for many reasons,” Van Mooy said. “One, the weather is a factor — and you’re isolated.

“I think we should have a reality show where we take 16 people and lock them up in about three rooms for a month,” he joked, adding that all the researchers onboard got along just fine.

Van Mooy says the research group has issued proposals asking to fund future research on the marine phosphorus cycle and the molecules involved, but is waiting for a response.

“We just scratched the surface,” Van Mooy said of the recent study. “The devil’s in the details and that’s what we’re going after next.”

Top image: Phytoplankton, or sea sawdust, play an an important role in the marine phosphorus cycle. (Credit: NOAA MESA Project)